This invention relates to an enhanced method and device for the isomerization of gasoline with a high benzene content.
It is known that the oil industry resorts to methods that are aimed at increasing the octane number of these gasolines by transforming the straight paraffins they contain into branched paraffins (or isoparaffins). In general, the charge to be treated is composed mostly of saturated hydrocarbons with five or six carbon atoms, as well as lower percentages of hydrocarbons with four or seven carbon atoms, and of benzene that, as we all know, is relatively difficult to separate from the other hydrocarbons with six carbon atoms.
In order to carry out the isomerization reaction, the charge to be treated is most often mixed with hydrogen and a possible recycle, it is then directed toward at least one reactor containing an appropriate fixed-bed catalyst. The temperature in this reactor usually ranges between 120 and 190xc2x0 C. When exiting the isomerization reactor (or reactors), the effluents are carried toward one or several separation columns. Often times, the isoparaffins are then separated from the non isomerized paraffins: in principle, the first are sent to the gasoline pool, whereas the second are possibly recycled in the reactor, in order to be transformed therein.
We also know that, under isomerization reaction conditions, the benzene that is present in the charge is hydrogenated, due to the presence of the hydrogen and the hydrogenating transition metals that enter into the composition of the isomerization catalysts. This results in a significant heat release in the upstream part of the isomerization reactor due to the exothermicity of this reaction, which is detrimental to the efficiency of the isomerization reaction.
This is why we generally use two isomerization reactors: the first reactor, in which, in addition to the isomerization of paraffins, the hydrogenation of traces of benzene present in the charge occurs, operates at a temperature slightly higher that the second, where the actual isomerization reaction is finished. Indeed, from a kinetic point of view, the isomerization reaction of paraffins is slower than the hydrogenation reaction of the benzene. Furthermore, the lower temperature in the second reactor is thermodynamically favorable to the creation of the desired branched products.
However, the benzene content of the isomerization charges remains relatively limited. Indeed, a rule commonly accepted suggests that a 1% increase of the benzene content in the charge to be treated produces an increase in temperature of 10xc2x0 C. inside the first isomerization reactor. Knowing that the reactions other than the hydrogenation of benzene already produce an increase in temperature in the range of 15xc2x0 C. on their own, the benzene content of the charge entering this reactor must then be limited to 4%. Beyond this content, the temperature inside the reactor is too high, which, in the end, not only harms the catalyst but also the unit, built to operate at a limited temperature. Furthermore, when the temperature is high, undesirable secondary reactions occur, such as for example, hydrocracking reactions of the charge.
However, it seems desirable to be able, in the isomerization unit, to treat charges with a benzene content that is much higher than the 4% generally accepted.
Indeed, considering the carcinogenic character of benzene, the current standards have a tendency to impose tougher and tougher limitations on the content of this composition in fuels.
An ingenious solution then consists in allowing, in the isomerization unit, in addition to the traditional charges, gasoline cuts that are rich in benzene, such as certain gasoline cuts resulting from catalytic reforming or cracking units: the benzene that is present in these cuts is thus hydrogenated in the isomerization unit, and this operation allows in fine to clearly reduce the benzene content of said cuts before carrying them to the xe2x80x9cgasoline poolxe2x80x9d, term by which we designate the overall bases used in manufacturing oil products.
However, this need to treat more benzene during isomerization comes up against the limitation tied to the temperature in the first reactor. Indeed it is still essential that this temperature be maintained at an average value in the range of 180xc2x0 C., above which the hydrocracking reactions, which are very exothermic, begin.
In order to rid oneself of this limitation, U.S. Pat. No. 5,003,118 proposes to introduce, upstream from the actual isomerization reactor (or reactors), a charge pretreatment reactor, specifically intended to execute the hydrogenation of the benzene present in this charge. In this way, the unit can treat charges with a higher benzene content. However, this solution has the disadvantage of being costly and requires building additional equipment. Furthermore, it is not absolutely flexible to the extent that the hydrogenation reactor is of no use in the case of a charge with a low benzene content.
Therefore, the intent of this invention is to propose a method for the isomerization of gasolines with a high benzene content which will make it possible to remedy the problems encountered in the prior art in an easy and inexpensive way.
This invention also intends to propose a particularly flexible isomerization method that will allow it to adapt quickly to the different charges brought to it.
With this in mind, the object of this invention is a method for the isomerization of a hydrocarbonic charge that contains a substantial quantity of paraffin base hydrocarbons with 5 or 6 carbon atoms and has a benzene content that is greater than or equal to 2% by weight, in which the charge to be treated passes, in the presence of hydrogen, at a total pressure that is greater than or equal to 10-105 Pa (10 bars) and at an average temperature ranging between 100 and 200xc2x0 C., through at least one reactor containing a paraffin base hydrocarbon isomerization catalyst, this method being characterized by the fact that, in the upstream part of the reaction zone, we introduce an adjunctive fluid that, at 40xc2x0 C. and under atmospheric pressure (1.0134.105 Pa), is in its gaseous phase and has a density that is less than or equal to that of the normal-pentane considered under the same conditions.
By density of a gaseous fluid, we mean the ratio between the density of this fluid and the density of dry air and with a normal carbon dioxide content, these two densities being measured under the same temperature and pressure conditions.
The density of the adjunctive fluid will be measured, at 40xc2x0 C. and under a pressure of 1.0134.105 Pa (1 atmosphere), by applying to said fluid one of the standardized methods of measurement described in the ASTM standard D1070-85 (R94). The same goes for the normal-pentane. The densities of said adjunctive fluid and of the pentane will be taken into consideration by reference to the same method of measurement.
According to the invention, the adjunctive fluid is in a gaseous phase at 40xc2x0 C. under a pressure of 1.0134.105 Pa (1 atmosphere). On the other hand, it is introduced in the upstream part of the reaction zone under temperature and pressure conditions that depend directly on the process conditions of the method. Based on its composition, it can then be in a liquid or gaseous phase or even in an intermediary phase.
Advantageously, said adjunctive fluid contains a substantial quantity of hydrogen and/or hydrocarbons that contain from one to five carbon atoms. Preferably, this adjunctive fluid contains a substantial quantity of hydrogen and/or hydrocarbons that contain from one to four carbon atoms, the preferred hydrocarbons being methane and ethane. This fluid can also, for example, consist of natural gas.
This fluid can also contain, in a smaller quantity, hydrocarbons with six or seven carbon atoms, and/or inert gases such as nitrogen, or any other appropriate light fluid.
Advantageously, said adjunctive fluid contains a substantial quantity of light compounds resulting from a fractionating tower located downstream from the isomerization unit.
The introduction of light adjunctive fluid creates, in the upstream part of reaction zone, a vaporization of a portion of the liquid fraction of the hydrocarbonic charge. This phenomenon is endothermic and helps to restore the heat balance within the reactor. We can thus compensate for the excess heat that emanates in the upstream part of the first reactor, following the hydrogenation of the benzene that is present in a higher quantity in the charge.
Furthermore, the drop in temperature within the reactor, resulting from the injection of said fluid, can be perfectly determined. Indeed, this injection results in a modification of the balances between the liquid fractions and the hydrocarbon vapors that make up the charge, balances that are governed by the thermodynamics laws.
Therefore, this drop in temperature depends solely on the following parameters: flow rates and compositions of the charge and of the adjunctive fluid, pressure, temperature, vaporized fraction of the charge upon entering the reactor and mass ratio between the hydrogen and the charge in the reaction mixture allowed in the unit. It can therefore be perfectly controlled and optimized based on the benzene content of the charge to be isomerized.
Thus, the method as set forth in the invention is precise and extremely flexible, to the extent that the flow rate of the adjunctive fluid can be regulated so as to compensate exactly for the rise in temperature caused by a benzene content that is greater than the usually accepted maximum content.
Thanks to the method as set forth in the invention, it now becomes possible to introduce, on charge of the isomerization unit, a greater fraction of gasoline rich in benzene, such as the light gasolines resulting from the catalytic reforming (xe2x80x9clight reformatexe2x80x9d) and/or cracking, alone or mixed with other charges. The benzene that is present in a large quantity in these cuts will be hydrogenated and therefore totally transformed into less toxic compounds within the isomerization unit. In this way, the benzene content of the xe2x80x9creduced gasoline cutxe2x80x9d, consisting of the isomerate, the light reformate and the heavy reformate, can be appreciably reduced. The impact in terms of toxicity reduction of the fuels is far from being insignificant. Also, the method as set forth in the invention turns out to be very advantageous for the units that do not have, downstream of the isomerization unit, a deisohexanizer (separation column for the sought branched paraffins and for straight paraffins), with the recycling of a cut that is rich in straight paraffins upstream of the isomerization unit.
Indeed, one of the objects of the recycling is to dilute the charge that has just been added to the unit, in order to lower its benzene content. Thanks to this invention, since it is possible to introduce charges with higher benzene contents into the unit, this dilution is no longer necessary. Therefore, for such units, the method as set forth in the invention represents a very valuable alternative to the costly installation of a deisohexanizer and a recycling system.
Lastly, unexpectedly so, the method as set forth in the invention has proved to allow for an increased recuperation of light hydrocarbons that contain at the most four atoms of carbon, and in particular propane and butane. These compounds are particularly interesting, all the more because they are amenable to beneficiation as GPL (Liquid Petroleum Gas), which is traditionally used as a fuel. The isomerization unit can thus be developed with a view to increase the production of these GPLs.
In accordance with the invention, the adjunctive fluid is injected in the upstream part of the reaction zone. This means that said fluid can be injected in the upstream part of the actual first reactor, and/or immediately upstream of the latter.
Preferably, the adjunctive fluid is injected in the upstream part of the first isomerization reactor, that is to say in the zone that extends from the introduction level of the reaction mixture (charge and hydrogen) in the reactor to half-way up the catalytic bed. Some adjunctive fluid can then be injected into the reactor zone contained between the introduction of the reaction mixture and the beginning of the dense bed of the catalyst. Advantageously, some adjunctive fluid can also be injected directly into the dense bed of catalyst, into the first half of the latter.
When adjunctive fluid is injected immediately upstream from the first reactor, it is introduced immediately, prior to the introduction of the reaction mixture (charge and hydrogen) in the first reactor, meaning after a complete preheating of this mixture and prior to the injection of the latter in the first reactor.
This injection of adjunctive fluid has a purely thermal effect and therefore does not have anything to do with the injection, in large quantity, of the hydrogen necessary for the actual isomerization reaction, which takes place, as known, upstream form the isomerization unit, meaning upstream from the heat exchangers in which the reaction mixture (charge and hydrogen) is reheated prior to being introduced in the reactor.
Therefore, this injection of adjunctive fluid is also carried out in a totally different location compared to the usual hydrogen injection upstream from the unit, and with a much weaker volume flow rate. In addition, its role is entirely different: this adjunctive fluid, which has a purely thermal effect, can consist of any light gas that is compatible with the method, whereas the hydrogen introduced upstream from the isomerization unit has a chemical effect at the level of the actual reaction.
This adjunctive fluid is advantageously injected at a flow rate of 5 to 150 Nm3 per m3 of charge to isomerize and, preferably of 5 to 60 Nm3 per m3 of charge. We are considering here the starting charge, prior to its mixture with hydrogen and prior to the heating of the reaction mixture thus obtained.
The effect that is sought is obtained: that the adjunctive fluid be injected at a temperature that is less than or greater than that of the reaction medium. Having said this, it is preferable to inject said fluid at a temperature that is less than or equal to that of the reaction medium, and, preferably, ranging between 20 and 180xc2x0 C.
When the isomerization unit consists of several successive reactors, a particularly advantageous alternative of the invention consists in recycling, immediately downstream from the first reactor, a cut that is rich in slightly branched paraffins with 5 or 6 carbon atoms and that generally contains naphthenes. This cut results, as known, from a fractionation that exists in the perfected isomerization units, is located downstream from the reactors, and separates the sought isoparaffins from the other compounds.
In the prior art, this recycling cut, which does not contain any benzene, is traditionally introduced upstream from the first reactor, in order not only to execute an additional passage through the isomerization unit, but also to dilute the charge that has just been admitted in the unit, so as to lower the benzene content of the combined charge so obtained.
Thanks to this invention, since it is possible to allow charges with a higher benzene content in the first reactor, this dilution is no longer necessary, so that only the new charge is treated in the first reactor.
A much better valorization results from this first reactor along with a considerable improvement of the isomerization reactions that take place therein. Indeed, the flow of the charge is weaker, which is translated by a reduction of the hourly volume speed, and therefore, a greater contact time between the charge and the catalyst. In addition, recycling this cut downstream from the first reactor makes it possible not to circulate the naphthenic compounds in this reactor, compounds which are known for being isomerization reaction inhibitors.
The invention also seeks to propose a device that will allow for the implementation of the method addressed above.
To this end, the object of the invention is an isomerization device for a hydrocarbonic charge that contains a substantial quantity of paraffin base hydrocarbons with 5 or 6 carbon atoms and a benzene content that is greater than or equal to 2% by weight, said device will have at least one reactor charged with a fixed bed catalyst, an incoming line into the reactor for the reaction mixture which consists of the charge mixed with a hydrogen rich gas, at least one heating system for said reaction mixture upstream from the reactor, and an evacuation line from the reactor for the charge that is enriched in isoparaffins; this device is characterized by the fact that, at least one means of introduction of an adjunctive fluid runs into the upstream part of the reaction zone, this fluid is in a gaseous phase at 40xc2x0 C., under atmospheric pressure (1.0134.105 Pa).
When the adjunctive fluid is introduced immediately upstream from the first isomerization reactor, an adjunctive fluid supply line runs into the incoming line of the first reactor of the reaction mixture (charge and hydrogen), between the most downstream reaction mixture heating device and the injection point of said mixture in the first reactor.
When the adjunctive fluid is introduced into the first reactor, at least one means of introduction of the adjunctive fluid leads into this reactor, upstream from the dense bed of the catalyst and/or into the first half of the latter. Said means can then consist of any known means that will allow, downstream from the injection of the actual charge, for the introduction of a light fluid in a reactor.
It can for example be a rod (or even a simple tube) with side slots or several holes, in order to allow for a better distribution of the adjunctive fluid. Preferably, we will use a diffuser that will make it possible to introduce the adjunctive fluid in a homogenous manner over the entire part of the reactor.
These means of introduction of the adjunctive fluid can lead into the reactor in several ways. According to one preferred mode of execution, said means emerge transversally in the reactor, considerably perpendicular to the axis of the latter.
According to another alternative, at least one means of introduction of the adjunctive fluid emerges in the reactor in a manner that is considerably parallel to the axis of the latter. Preferably, said means then penetrates into the reactor through the hole that allows for the introduction of the reaction mixture. This solution is particularly advantageous in the scope of modernization of existing units, since it avoids having to go through the wall of the reactor.
The invention does not relate to catalysts that are likely to interfere with its implementation. We can indeed use any known catalyst that shows an activity for the isomerization of straight paraffins into branched paraffins. In this field, many catalysts are known to the person skilled in the art. They usually contain one or several acid type functions, as well as a hydrogenating function (hydrogenating transition metal). We can name, as a non restrictive example, aluminum chloride base catalysts (or catalysts with other halogenated sites) and catalysts that contain one or several metals of group VIII of the Periodic table of the elements.